A dual master cylinder won't stop you any faster, except when you have a brake failure. I was driving my 65 Mustang convertible, a bolt in the exhaust hanger backed out, the exhaust fell on the rubber brake line, burned through it, and the entire brake system failed. Next was a head-on crash at 45 into a stopped truck, and a trip to the emergency room.
With a dual master cylinder, I would have still had front brakes when the rear brake circuit failed.
Jim - Erie Colorado, 1957 Country Sedan